Nested tables on only some records

See if this helps tloF2D5U9E5.

NOTE: EDITED - See more streamlined solution below

You need to parse the data out of the Table field, and Text.Combine can do it. Here is the code. Don't worry about all of the stuff at the top. This is just me creating a table that I think mimics your scenario. It is the Query1 line in the steps when you paste it in a blank query (directions on doing this below)

let
    Query1 = #table(
            {"ID", "Name", "City"},
                {
                    {123, "Alice", "Wonderand"},
                    {456, "Bob", "Wonderland"},
                    {789, "Jeff", #table(
                        {"Data"},
                        {
                            {"One Line"},
                            {"Two Line"},
                            {"Three Line"}
                        }
                    )
                    }
                }
          ),
    #"Added Custom" = 
        Table.AddColumn(
            Query1, 
            "Custom", 
            each
                if Value.Is([City], type table) 
                then Text.Combine([City][Data], ", ") 
                else [City])
in
    #"Added Custom"

Here is the result:

The key is this code:

if Value.Is([City], type table) 
then Text.Combine([City][Data], ", ") 
else [City])

If the value in the City column is a table type, then use Text.Combine([City][Data}, ", ") - what this does is it gets the City table here, and the Data column (yours will be named differently) and returns the values in it as a list. Text.Combine() will take a list and combine them using whatever separator you want.

If the City column is not a table, then it just pulls the text.

STREAMLINED VERSION:
This will use the Replace feature, so you don't generate a new column then have to delete it.

let
    Query1 = #table(
            {"ID", "Name", "City"},
                {
                    {123, "Alice", "Wonderand"},
                    {456, "Bob", "Wonderland"},
                    {789, "Jeff", #table(
                        {"Data"},
                        {
                            {"One Line"},
                            {"Two Line"},
                            {"Three Line"}
                        }
                    )
                    }
                }
          ),
    #"Replace Values" =
        Table.ReplaceValue(
            Query1,
            each if Value.Is([City], type table) then [City] else null,
            each Text.Combine([City][Data], ", "),
            Replacer.ReplaceValue,
            {"City"}
        )
in
    #"Replace Values"

Before:

After:

I don't think there is a performance difference, and the code is a bit harder to understand because Table.ReplaceValue has some redundant stuff if working on one column, but it does prevent having to clean up extra columns later. It is the same basic logic though:
Table.ReplaceValue(
     Query1,                                                                           The table you are working on
     each if Value.Is([City], type table) then [City] else null,   if the City column is a table then use that, otherwise null.
     each Text.Combine([City][Data], ", "),                              Combine data from [City] table. if null, no replace done.
     Replacer.ReplaceValue,                                                   Replacement type. You cannot use Replacer.Replacetext.
     {"City"}                                                                            The column you are working on.
)

How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.